A spinal cord stimulator is a device used to exert pulsed electrical signals to the spinal cord to control chronic pain. Further applications are in motor disorders. The lumbar spinal cord is a preferred target for the control of spinal spasticity[1][2] or augmentation of standing and stepping capabilities.[3][4][5][6] Spinal cord stimulation (SCS), in the simplest form, consists of stimulating electrodes, implanted in the epidural space, an electrical pulse generator, implanted in the lower abdominal area or gluteal region, conducting wires connecting the electrodes to the generator, and the generator remote control. SCS has notable analgesic properties and, at the present, is used mostly in the treatment of failed back surgery syndrome, complex regional pain syndrome and refractory pain due to ischemia.

FBSS, classified as mixed pain syndrome (neuropathic and nociceptive), is the persistent or recurrent pain, mainly involving the lower back and/or legs after successful spinal surgery. It affects about 40% of patients who undergo spinal surgeries. Several studies showed overall efficacy of the SCS for FBSS.[7][8][9][10][11]

SCS is also indicated in the treatment of inoperable ischemic limb pain.[12] Furthermore, this technique is studied in various applications. For instance, it has been shown to modulate the function of sympathetic nervous system and increase norepinephrine release in refractory angina pectoris,[13] decreasing the probability of angina attack. SCS units have been used to treat patients with frequent migraines. The electrodes are implanted in the bilateral suboccipital region.[14] See[15] and[16] for a more complete list and references of further applications.

SCS is analgesia on demand. It is a useful option when other forms of therapy fail. It reduces pain medication and side effects. It is effective in about 50–70% cases. It is an invasive procedure, so it can have associated complications such as infection, bleeding, and dural puncture. It has the risk of disconnection or equipment failure.[15]

Complications with SCS range from simple easily correctable problems to devastating paralysis, nerve injury and death. However, in a 7-year follow-up, the overall complication rate was 5-18%. The most common complications include lead migration, lead breakage, and infection. Other complications include haematomas (subcutaneous or epidural), cerebrospinal fluid (CSF) leak, post dural puncture headache, discomfort at pulse generator site, seroma and transient paraplegia. Hardware-related complications such as electrode migration, fractured electrodes, and rotation of pulse generator are also reported.[16]

The neurophysiologic mechanisms of action of spinal cord stimulation are not completely understood. Linderoth and others[17][18] have noted that the mechanism of analgesia when SCS is applied in neuropathic pain states may be very different from that involved in analgesia due to limb ischemia. In neuropathic pain states, experimental evidence show that SCS alters the local neurochemistry in dorsal horn, suppressing the hyperexcitability of the neurons. Specifically, there is some evidence for increased levels of GABA release, serotonin, and perhaps suppression of levels of some excitatory amino acids, including glutamate and aspartate. In the case of ischemic pain, analgesia seems to derive from restoration of the oxygen demand supply.[16] This effect could be mediated by inhibition of the sympathetic system, although vasodilation is another possibility. It is also probable that a combination of the two above mentioned mechanisms is involved.[15]

The generator, implanted subcutaneously, could be a complete pulse generator module with its own battery or only a radio frequency (RF) receiver. The former case, usually called implantable pulse generator or IPG, has a battery of its own and could come with rechargeable battery which can be charged externally via a wireless power charger so that it does not need to be replaced surgically when it loses charge.

The RF receiver on the other hand is externally driven by a transmitter from which it gets its power and pulses. This external transmitter has a battery which can be easily replaced. RF receivers have traditionally been used for patients who require high power settings that would quickly deplete a primary-cell IPG.[15]

The patient is also provided with a remote control to turn on and off the stimulator, and depending on the device and the surgeon’s preference, can change the programming of the stimulation patterns. The surgeon has a programming device that could be used to modify a wide range of stimulation settings of the RF generator.[15]

The electrodes, which consist of an array of leads, could be percutaneous type or paddle type. Percutaneous electrodes are easier to insert in comparison with paddle type, which are inserted via incision over spinal cord and laminectomy.[16]

SCS procedure involves careful placement of electrodes in the epidural space, a trial period (which takes between 5–7 days), and, if the results of pain relieving was satisfactory in the trial period, anchoring the electrodes to the interspinal ligaments, positioning and implantation of the pulse generator, tunneling and connection of the connecting wires, programming the system for the special pattern of stimulation and performing required postoperative cares.[15]

The representation of the dermatomal level in the dorsal columns of the spinal cord is much higher than the corresponding vertebral level. For instance, the sweet spot for sciatic pain (dermatomal level L5/S1) is around T10 nerve.[15] See dermatome and Spinal cord segments.

The IPG or the RF unit is usually implanted in the lower abdominal area or in the posterior superior gluteal region. It should be in a location that patients can access with their dominant hand for adjustment of their settings with the patient-held remote control. The decision to use a fully implantable IPG or an RF unit depends on several considerations. If the patient’s pain pattern requires the use of many electrodes with high power settings, an RF unit should be used. The IPG battery life will largely depend on the power settings utilized, but the newer IPG units will generally last several years at average power settings.[15]

Programming involves selecting the electrode stimulating configuration, adjusting the amplitude, width and frequency of electrical pulses. Amplitude indicates the intensity of stimulation. This is delivered in milliamperes or volts depending on the system used. Lower voltage or current is chosen for peripheral nerves and paddle leads. Pulse width usually varies from 100 to 400 us. Widening the pulse width will also broaden the area of paraesthesia. Frequency of pulse wave is usually between 20 and 120 hertz. It is an individual preference: some patients choose low frequency beating sensation whereas others prefer high frequency buzzing.[15]

Selection of lowest possible setting on all parameters is important in conserving battery life in non-rechargeable models of SCS. Cycling of stimulation is also employed to save battery life. Changing of stimulator program may have to be undertaken during the course of therapy and follow-up.[15]

Appropriate patients for neurostimulation implants must meet the following criteria: the patient has a diagnosis amenable to this therapy, the patient has failed conservative therapy, significant psychological issues have been ruled out, and a trial has demonstrated pain relief. A trial period of stimulation over a period of 5–7 days should follow the psychiatric evaluation to demonstrate its effectiveness. This part of the protocol is important because of the cost of the equipment and the invasive nature of the procedure. The trial is considered successful if the patient achieves more than a 50% reduction in pain.[16]

Electrotherapy of pain by neurostimulation began shortly after Melzack and Wall proposed the gate control theory in 1965.[19] This theory proposed that nerves carrying painful peripheral stimuli and nerves carrying touch and vibratory sensation both terminate in the dorsal horn (the gate) of spinal cord. It was hypothesized that input to the latter could be manipulated to “close the gate” to the former. As an application of the gate control theory, Shealy et al. implanted the first spinal cord stimulator device directly on the dorsal column for the treatment of chronic pain[20] and in 1971, Shimogi and colleagues first reported the analgesic properties of epidural spinal cord stimulation.[21] Since then this technique has undergone numerous technical and clinical developments.[16]

The cost effectiveness of spinal cord stimulation in the treatment of chronic back pain was evaluated by Kumar and colleagues in 2002.[22] They examined 104 patients with failed back surgery syndrome. Of the 104 patients, 60 were implanted with a spinal cord stimulator. Both groups were monitored over a period of five years. The stimulation group annual cost was $29,000 versus $38,000 in the other group. 15% returned to work in the stimulation group versus 0% in the other group. The higher costs in the nonstimulator group were in the categories of medications, emergency center visits, x-rays, and ongoing physician visits. See also[23] for another study of cost effectiveness. See[16] for list of such studies on cost effectiveness in various applications of SCS.

SCS is finding its way to be applied to Parkinson’s disease.[24][25] Recently, a case study demonstrated the first successful result of this technique in a patient with Parkinson's disease.[26] More complex and power efficient microprocessor based equipment increasing the battery life could be developed. Closed loop bio-feedback systems which communicate and record neural responses following spinal cord stimulation could be applied and utilized[27] In the future, it might be possible to combine SCS with implanted drug delivery systems to produce synergistic effects minimizing side effects and complications. Strong evidence is still lacking for SCS which may emerge in near future following robust research studies complimenting the rapid technological advances that is taking place in the field of SCS.[15]